The present invention relates to a nuclear reactor start-up method after the cold shut-down and, more particularly, to a start-up method which is suitable for ensuring thermal-hydraulic stability of a nuclear reactor which is in the natural circulation state or in the state near it when the reactor is started up.
Generally, boiling water reactors have a characteristic such that a range (stability allowance) in which the nuclear reactor can be safely run at a low pressure or in the natural circulation state is narrow. Therefore, it is necessary to run the reactor in sufficient consideration of the stability allowance when the reactor is started up.
FIG. 1 shows the relationship between a power of a nuclear reactor in the natural circulation state and a reactor core flow and an instability occurrence range based on conventional knowledge. It can be seen from this diagram that the range where the nuclear reactor can be safely run is narrow when a pressure and a flow are low. This is because the natural circulation flow is based on the balance between the buoyancy which is due to the difference between the density of coolant in the reactor core at a high temperature and that in the flow channel outside the reactor core at a low temperature, and the frictional loss of the vapor-liquid two-phase flow in the reactor core. When the pressure and flow are low, there is a tendency such that the variation in friction loss promotes the variation in natural circulation flow, causing the natural circulation flow to become unstable. This point will be described later.
In order to enhance the stability allowance, as shown in FIG. 2, a recirculating system 5 is provided in the outside of a pressure vessel 1 so as to forcedly circulate a cooling water 3. FIG. 3 is a start-up map showing a start-up method of a forced circulation type nuclear reactor. In this method, power is increased while the cooling water 3 is circulated at a pump speed of 20% of the rated speed. When the reactor is started up, as shown in FIG. 4, the use of such a method enables the stability allowance to be enlarged with respect to that in the case where the reactor is run in accordance with the natural circulation curve.
FIG. 5 shows an alternate expression of running curves of FIG. 4 in terms of a reactor inlet subcooled temperature .DELTA.T.
The subcooled temperature (.DELTA.T) is defined by the difference between a saturation temperature T.sub.B (a boiling point at a certain pressure P) of the cooling water 3 and an actual temperature T.sub.A of the cooling water 3, namely, by the following expression (1). EQU .DELTA.T=T.sub.B -T.sub.A ( 1)
Since the boiling point T.sub.B depends on the pressure of the cooling water 3, we will have EQU .DELTA.T=.DELTA.T (P) (2)
In addition, the reactor core inlet subcooled temperature, therefore, denotes the subcooled temperature .DELTA.T of the cooling water 3 flowing into the reactor core.
All of the above-mentioned conventional nuclear reactor start-up methods consider only the instability range regarding the variation of the natural circulation flow due to the variation in frictional loss mentioned above which has been known so far. However, in the case where the nuclear reactor is in the natural circulation state accompanied with generation of voids in the reactor core or in the state near the natural circulation (including the slight forced circulation flow) it has been found that another instability different from the mechanism of the above-mentioned instability exists. Thus, there is a problem such that the sufficient stability allowance cannot be ensured by the conventional nuclear reactor start-up method.